It is customary in rolling mills to apply a coolant to the rolls and/or the work to hold the temperature of the work within reasonable limits regardless of the heat generated during rolling. The coolant is usually water or is water-based. It has been customary to also include a rolling lubricant, for example, in the form of an oil-water emulsion.
However, when water-based coolant contacts aluminium strip, it reacts with the aluminum to cause staining of the strip surface, which may inhibit the action of the rolling lubricant, even if applied separately from the coolant. The result is that the reduction effected in the mill is non-uniform across the strip width so that strip with poor flatness is produced. Further, the hardness of the stained areas differs from that of the remainder of the strip and this difference causes unequal reduction in any subsequent rolling operation and further loss of flatness. Lastly, the appearance of the rolled material is marred.
To overcome these problems, European Patent Specification No. 0160381A proposes a method of rolling metal in a rolling mill in which liquid coolant is applied to a roll or rolls on the ingoing side of the mill within a casing or casings, unwanted egress of coolant from the casing or casings being prevented by the use of contact seals engaging the roll or rolls. Rolling lubricant is independently applied to the work and/or the work rolls at the ingoing side of the mill outside the casing or casings.
This method has several advantages. Thus, as the coolant is applied on the ingoing side of the mill in a casing or casings from which unwanted egress of coolant is effectively prevented, because of the direction of rotation of the rolls, there is minimal possibility of coolant being transferred through the nip between the work rolls and their back-up rolls and thence to the work at the outgoing side.
Secondly, contact seals are used, which are more effective than air seals in preventing escape of moisture. Their use is possible because the rolls at the ingoing side of the mill are at a reasonably low temperature and because the contact seals are lubricated by the coolant. No coolant mist is generated and there is no escape route for the mist even if it were generated. The contact seals further act as cleaners for the rolls and prevent particulate material being carried into the roll gap by the rolls and damaging the rolls and the work.
Thirdly, rolling lubricant is applied separately from the coolant at the ingoing side of the mill and outside the casing or casings. The separate application of rolling lubricant is essential because of the effectiveness of the contact seals but, apart from that, it enables the lubricant to be distributed more evenly, makes possible better control of the lubricant, and can result in better efficiency of lubricant usage.
However, it will be appreciated that the use of contact seals between the rolls and the casing arrangement used requires the provision of effective end seals at the ends of the rolls. The end seals must provide effective contact with both a work roll and its associated back-up roll extending over the region of the nip between the two rolls.
Inevitably the contact seals are subject to wear and will require repositioning or replacement when worn in order to ensure effective operation. Shut down of the rolling mill for seal maintenance is clearly economically undesirable. Furthermore, even on shut down, the positioning of the seals in close proximity to the mill rolls makes maintenance access difficult.
Regular roll maintenance, for example, for regrinding of the work rolls, is an accepted feature of rolling mill operation, whereby the rolls are withdrawn sideways from the mill to a roll change car for transfer to a regrinding or storage station and fresh rolls are then moved into position to define the roll gap.